The present invention relates generally to an open superconductive magnet used to generate a uniform magnetic field as part of a magnetic resonance imaging (MRI) system, and more particularly to such a magnet having shielding to protect the area around the magnet from stray magnetic fields originating from the magnet.
MRI systems employing superconductive magnets are used in various fields such as medical diagnostics. Known superconductive magnets include liquid-helium-cooled and cryocooler-cooled superconductive magnets. Typically, the superconductive coil assembly includes a superconductive main coil surrounded by a thermal shield surrounded by a vacuum enclosure. A cryocooler-cooled magnet preferably also includes a cryocooler coldhead externally mounted to the vacuum enclosure, having its first stage in thermal contact with the thermal shield, and having its second stage in thermal contact with the superconductive main coil. A liquid-helium-cooled magnet preferably also includes a liquid-helium container surrounding the superconductive main coil and a second thermal shield which surrounds the liquid-helium container and which itself is surrounded by the thermal shield.
Known superconductive magnet designs include closed magnets and open magnets. Closed magnets typically have a single, tubular-shaped superconductive coil assembly having a bore. The superconductive coil assembly includes several radially-aligned and longitudinally spaced-apart superconductive main coils each carrying a large, identical electric current in the same direction. The superconductive main coils are thus designed to create a magnetic field of high uniformity within a spherical imaging volume centered within the magnet's bore where the object to be imaged is placed. A single, tubular-shaped superconductive shielding assembly may also be used to prevent the high magnetic field created by and surrounding the main coils from adversely interacting with electronic equipment in the vicinity of the magnet. Such shielding assembly includes several radially-aligned and longitudinally spaced-apart superconductive shielding coils carrying electric currents of generally equal amperage, but in an opposite direction, to the electric current carried in the main coils and positioned radially outward of the main coils.
Open magnets typically employ two spaced-apart superconductive coil assemblies with the space between the assemblies allowing for access by medical personnel for surgery or other medical procedures during MRI imaging. The patient may be positioned in that space or also in the bore of the toroidal-shaped coil assemblies. The open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design. Open magnets may be shielded passively, but that would take away the openness of the magnet. Shielding the room (instead of the magnet) is too expensive to consider in many installations.